The invention relates to the general field of accelerometers with particular reference to Z-axis units and methods for their manufacture.
Accelerometers have wide applications such as for inertial navigation systems, automotive safety, and missile control. Z-axis accelerometers can be used to control side air bags, vehicle control and multi-axis sensing systems. Normally, z-axis accelerometers are fabricated using bulk micro-machined technology or stacked thin films. Such devices have large size and need double-side bonded three wafer processes. This can lead to stress control problems in the films which, in turn, causes sticking problems during release.
The basic principle underlying the operation of units of this type is schematically illustrated in FIG. 1. Two plates, 14a and 14b are attached by springs 13 to support posts 12 and their capacitance relative to the upper surface of substrate 11 is monitored. While the device accelerates in direction A, plates 14a and 14b are drawn closer to the upper surface of 11 and their mutual capacitance decreases in proportion to the rate of acceleration. Similarly, during acceleration in the -A direction, the capacitance increases.
The arrangement shown in FIG. 1 has the merit of providing a high level of sensitivity. However it has the serious limitation that it cannot distinguish capacitance changes due to acceleration from changes arising from other causes such as temperature, and other possible systematic errors. Since both capacitor plates respond to acceleration by moving in the same direction, a differential design in which the capacitance changes differently (preferably oppositely) for the two plates is not feasible.
A routine search of the prior art was performed. The following references of interest were found:
In U.S. Pat. No. 5,576,250, Diem et al. show how to fabricate off-set parallel plates for use in a sensor. The approach used is to form beams by etching and then filling trenches, said filling material then becoming the beam. MacDonald et al. (U.S. Pat. No. 5,770,465) show a trench fill masking technique while Andersson (U.S. Pat. No. 5,723,790) teaches an accelerometer with multiple cantilever beams free to move in specific directions, said motions being detected by means of piezoresistive sensors. Ishida et al. (U.S. Pat. No. 5,830,777) and Delapierre (U.S. Pat. No. 4, 776,924) show other accelerometers processes.
It has been an object of the present invention to provide a Z-axis accelerometer design.
Another object of the invention has been to provide a process for manufacturing said accelerometer.
A further object has been that said accelerometer be insensitive to motion that arises from any cause other than acceleration (such as temperature changes).
These objects have been achieved by measuring a change in capacitance when one plate is fixed and one is mobile (free to accelerate). Unlike prior art designs where such changes are caused by variations in the plate separation distance, in the design of the present invention the plate separation distance is fixed, it being the effective plate area that changes with acceleration. A key feature is that the basic unit is a pair of capacitors. The fixed plates in each case are at the same relative height but the mobile plates are offset relative to the fixed plates, one mobile plate somewhat higher than its fixed plate with the other mobile plate being somewhat lower. Then, when the mobile plates move (in the same direction), one capacitor increases in value while the other decreases by the same amount. This differential design renders the device insensitive to sources of systematic error such as temperature changes. A process for manufacturing the design is described.